Welding method for bimetal composite pipes for acidic environment

A welding method for bimetal composite pipes for an acidic environment, comprising the following steps:

• (1) machining of a stepped notch
• first, preparing a finished bimetal composite pipe formed by hydroforming and cleaning a surface and an end face, and then cutting off a certain range of one side of a pipe end near a liner pipe to form the stepped notch,
• wherein the stepped notch has a height a of 2.5~4 mm and a depth b of 8~12 mm, and a wall thickness L2 of the liner pipe<the height a of the notch<a wall thickness (L1+L2) of 25% bimetal composite pipe;
• (2) preparation of cladding powder
• mixing carbon powder, chromium powder, niobium powder, silicon powder, molybdenum powder, manganese powder, iron powder and nickel powder evenly to obtain surfacing layer powder and isolation layer powder respectively,
• wherein the surfacing layer powder and the isolation layer powder, by mass percentage, each comprise 0.01%~0.03% of carbon powder, 21%~22% of chromium powder, 3%~3.5% of niobium powder, 0.01%~0.1% of silicon powder, 8%~10% of molybdenum powder, 0.1%~0.3% of manganese powder, and 1%~5% of iron powder, with a balance being nickel powder;
• and
• the surfacing layer powder and the isolation layer powder have a particle size of 5~50 μm and a sphericity of 0.7~0.9;
• (3) cleaning and preheating of the composite pipe
• performing cleaning and preheating treatment on the bimetal composite pipe with the stepped notch machined in step (1);
• (4) laser cladding surfacing
• loading the surfacing layer powder in step (2) into a powder feeder of laser cladding equipment, keeping the composite pipe rotating, cladding the surfacing layer powder in the stepped notch in step (1) by using a synchronous laser cladding process in a cladding direction from an inside to an outside, pausing the laser cladding equipment after a surfacing layer is formed, wherein a roundness deviation range of an inner wall surface of the surfacing layer is 0.5%~1%;
• and polishing an end face of the surfacing layer so that the end face of the surfacing layer is flush with the end face of the composite pipe;
• (5) groove designing and machining
• machining a V-shaped groove on the end face of the composite pipe after the surfacing in step (4),
• wherein for the V-shaped groove, a blunt edge length c is 3~6 mm, a blunt edge thickness d is 2~2.5 mm, and a single-sided groove angle β is 30°±3°;
• (6) groove cleaning and preheating
• performing cleaning and preheating treatment on the V-shaped groove of the end face of the composite pipe in step (5);
• (7) adding of an isolation layer to the groove
• loading the isolation layer powder in step (2) into the powder feeder of the laser cladding equipment, keeping the composite pipe rotating, cladding the isolation layer powder on an inclined surface of the V-shaped groove in step (5) by using the synchronous laser cladding process in the cladding direction from the inside to the outside, and stopping the laser cladding equipment after the isolation layer is formed,
• wherein a height of the isolation layer is uniform and consistent, the single-sided groove angle β is maintained at 30°±3°, and a thickness e is 2~5 mm;
• (8) groove cleaning and fit-up
• performing quality inspecting and a cleaning on grooves of two processed to-be-welded composite pipes to ensure that the grooves and surrounding areas thereof are intact, bright and clean, and then performing groove fit-up;
• (9) welding and filing of a base layer
• performing base layer welding on a bottom of the groove by using tungsten inert gas welding under inert gas protection to form a base weld layer, then performing fill welding on the base weld layer by using the tungsten inert gas welding to form a first fill layer, a second fill layer and a third fill layer;
• and
• (10) welding of a base layer cover
• performing cover welding on the base layer by using shielded metal arc welding to form a cover layer.